The pupil is the circular orifice in the center of the iris in the eye. Light rays enter the eye through the pupil and travel to the retina, which is the light sensitive membrane at the posterior of the eye. Changes in pupil size can be an indicator of disease. Unequal pupil sizes can also be an indicator of disease. About twenty percent of the population has anisocoria, which is a condition where pupil sizes are unequal. For the other eighty percent of the population, unequal pupil size would be an indicator of disease that can be measured and acted upon in various clinical settings.